Prior fuel injection systems which may be used with, for example, diesel engines, have typically been of the pump-line-injector type or the unit injector type. A pump-line-injector fuel injection system includes a main pump which pressurizes fuel to a high level, e.g., on the order of 103 to 138 MPa (about 15,000 to 20,000 p.s.i.), and individual fuel injectors which are coupled by fuel supply lines to the pump. In a unit injector system, a low-pressure pump delivers fuel to a plurality of unit injectors, each of which includes means for pressurizing the fuel to a relatively high value, again on the order of 103 to 138 MPa (about 15,000 to 20,000 p.s.i.) or greater.
One type of unit injector system is known as a hydraulically actuated, electronically-controlled unit injector (HEUI) and is disclosed in Glassey U.S. Pat. No. 5,191,867. Actuating fluid in the form of engine oil is pressurized to an intermediate pressure of, for example, 20.7 MPa (3,000 p.s.i.) and is supplied to each unit injector. An engine control module develops injector actuation signals which are supplied to a solenoid winding of each injector. When a solenoid winding is energized by the ECM, a poppet is moved by the solenoid to allow the pressure actuating fluid flow to an intensifier chamber. In response to the admittance of pressurized actuating fluid to such chamber, an intensifier piston is displaced in a direction which pressurizes fuel disposed in a high pressure chamber. The high pressure chamber is in fluid communication with a chamber containing an elongate check which is spring biased against a sealing surface to isolate the check chamber from a combustion chamber of the engine. When the pressure in the check chamber exceeds a valve opening pressure determined by the spring force exerted on the check, the check is lifted, thereby spacing the check tip away from the sealing surfaces and permitting pressurized fuel to escape into the associated engine combustion chamber through one or more injector nozzle orifices. Injection is ended by deenergizing the solenoid winding, thereby causing the poppet to move to a position to isolate the intensifier chamber from the pressurized actuating fluid. The pressure of the fuel in the high pressure chamber abruptly drops, thereby permitting the spring to close the check against the sealing surface and terminating further fuel injection.
While the HEUI injection apparatus has been useful to control the admittance of pressurized fuel into an associated engine combustion chamber relative to approximately top dead center (TDC), such apparatus is only indirectly controlled, i.e., the motive force for moving the injector check is provided by the pressurized fuel itself rather than a directly controllable motive power source. Accordingly, the degree of controllability required to desirably reduce particulate and gaseous emissions in accordance with regulatory agency standards is minimal.
Gibson et al. U.S. patent application Ser. No. 08/172,881 discloses a fuel injector having a force-balanced check which is movable between open and closed positions by means of a low-force actuator. This fuel injector provides a high degree of controllability and is capable of use with high fuel injection pressures, thereby permitting a desirable reduction in undesirable exhaust emissions.
SAE paper 910252 by Miyaki et al. discloses a fuel injector utilizing a three-way valve to control injection by controlling the application of fluid pressure from a high pressure source to ends of a check. The injector is designed to minimize biasing forces resulting from fluid pressure differentials tending to urge the three-way valve toward either the first or second travel limit positions. This is accomplished by incorporating an inner valve slidably fitted inside an outer valve which in turn is slidably fitted inside a valve body. The clearance between the inner and outer valve and between the outer valve and the valve body provide leakage paths which are continuously subjected to the high supply pressure. For most operating conditions of the intended diesel engine application the resulting leakage exceeds the amount of fuel injected into the associated engine cylinder, thus constituting a significant reduction in the efficiency of the injection system.